Hitherto, a light-emitting diode (abbreviated as LED) and a laser diode (abbreviated as LD) having a Group III-V compound semiconductor (e.g., Group III nitride semiconductor) light-emitting layer containing a Group V constituent element such as nitrogen (symbol of element: N) are well known as light-emitting devices for principally emitting blue to green light (see, for example, Patent Document 1). Conventionally, such LEDs emitting short-wavelength visible light include a light-emitting layer generally composed of a gallium indium nitride mixed crystal (compositional formula: GaYInZN: 0≦Y, Z≦1, Y+Z=1) layer (see, for example, Patent Document 2).
(Patent Document 1)
Japanese Patent Application Laid-Open (kokai) No. 49-19783
(Patent Document 2)
Japanese Patent Publication (kokoku) No. 55-3834
(Patent Document 3)
Japanese Patent Application Laid-Open (kokai) No. 2-288388
(Patent Document 4)
Japanese Patent Application Laid-Open (kokai) No. 2-275682
(Non-Patent Document 1)
Book written and edited by Isamu AKASAKI, “Group III-V Compound Semiconductors,” published Baifukan Co., Ltd., 1st edition, Chapter 13, (1995).
In general, an n-type light-emitting layer composed of a Group III nitride semiconductor is joined to form a heterojunction to a cladding layer for supplying carriers (electrons and holes) which cause radiation-recombination to emit light in the light-emitting layer (see, for example, Non-Patent Document 1). Conventionally, the p-type cladding layer for supplying holes to the light-emitting layer is generally composed of aluminum gallium nitride. (AlXGaYN: 0≦X, Y≦1, X+Y=1) (see Non-Patent Document 1).
Another known technique for fabricating a light-emitting device includes providing, on a p-type AlXGaYN (0≦X, Y≦1, X+Y=1) layer, a p-type boron phosphide (BP) layer serving as a contact layer for forming an Ohmic electrode thereon, the BP layer being doped with a p-type impurity element such as magnesium (symbol of element: Mg) (see, for example, Patent Document 3). For example, a laser diode (LD) is fabricated through provision of an Mg-doped p-type BP layer serving as a contact layer which is joined to a superlattice structure layer consisting of an Mg-doped boron phosphide layer of a zinc-blende structure and a Ga0.4Al0.6N layer (see, for example, Patent Document 4).
Boron-phosphide-based semiconductor (typically boron phosphide) crystals generally have a crystal form of sphalerite. Since such cubic crystals have a degenerated valence band, a p-conduction-type crystal layer is readily formed as compared with a hexagonal crystal (Japanese Patent Application Laid-Open (kokai) No. 2-275682, see the aforementioned Patent Document 4). However, doping a boron-phosphide-based Group III-V compound semiconductor crystal layer with a Group II impurity element (e.g., Mg) does not always result in formation of a low-resistive p-type conductive layer having low and constant resistivity. Furthermore, Mg may serve as a donor impurity with respect to boron phosphide, and in some cases, a high-resistive or an n-conduction-type boron-phosphide-based semiconductor layer is formed through doping with Mg.
For example, in production of a stacked structure having a p-type BP layer provided on the Mg-doped p-type GaAlBNP Group III nitride semiconductor mixed crystal layer disclosed in Patent Document 3, a low-resistive p-type BP layer is not reliably formed even though the BP layer is intentionally doped with Mg. Therefore, pn-junction compound semiconductor light-emitting devices (such as LEDs) including a p-type boron-phosphide-based semiconductor layer fail to attain low forward voltage (Vf).